The borough council's planning committee was discussing highly controversial plans to build 900 homes and widen a road through woodland in Titnore Lane, Durrington, Worthing.
The intervention began after meeting chairman, Tory John Livermore, refused to take any more questions from the public at the packed hall, which was being used as an emergency council chamber because of the massive local interest.
While security staff ejected one protester, others occupied the stage, unfurling a large banner and forcing councillors to flee.
Police eventually removed all the protesters from the hall, after a half hour interruption, while chanting continued outside the front doors. There were no arrests